Le 17/11/12 15:22, Hadley Wickham a écrit :
Hi all,
Am I doing something wrong with the functions below, or has this
behaviour not yet been implemented in Rcpp?
cppFunction('std::string first_char(const CharacterVector x) {
return x[0];
}')
Ah.
> cppFunction('std::string first_char(CharacterVector x) {
+ return x[0] ;
+ }')
filee454ce15417.cpp: In function ‘std::string
first_char(Rcpp::CharacterVector)’:
filee454ce15417.cpp:7: error: conversion from
‘Rcpp::internal::string_proxy<16>’ to non-scalar type ‘std::string’
requested
Because of the way character vectors are implemented, we had to use a
proxy mechanism. so the result of the operator[] is a:
string_proxy<16>
which has these two conversion operators:
operator SEXP() const
this just retrieves the underlying CHARSXP using
STRING_ELT
operator /*const */ char*() const
this also applies CHAR() to it to get a C string.
I think we tried having an automatic conversion to std::string, but this
failed and the compiler was confused.
Anyway. Short of automatic conversion, you can use a forced one:
cppFunction('std::string first_char(const CharacterVector x) {
return std::string( x[0] );
}')
it works because of "only distance 1" conversion to go from string_proxy
and something (in that case char*) the ctor of string will accept.
cppFunction('CharacterVector miss_c() {
return CharacterVector::create(NA_STRING);
}')
I've seen this too and was about to find my ways to a fix.
This works though:
cppFunction('CharacterVector miss_c() {
CharacterVector out(1) ;
out[0] = NA_STRING ;
return out ;
}')
Not as convenient. :-/
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